The present invention relates to wafer surface wetting for metallization, more particularly, relates to pre-wetting wafer surfaces.
Advanced interconnections of ultra large scale integrated (ULSI) circuits are fabricated in part by electrochemically or chemically (also known as “electrolessly”) depositing metallic materials, typically copper, onto wafer surface from one or more electrolyte solutions. The deposition process, beginning with a dry wafer immersing into an electrolyte bath, or with sprays of an electrolyte solution onto a dry wafer, may encounter several problems. One severe problem is the incomplete wetting of electrolyte on the wafer surface to be deposited, for that no deposition occurs in areas where electrolyte is not in contact with the wafer surface. The wetting related defects formed by this mechanism, often manifest as pits on deposited films and large voids or miss-filled segments in vias and trenches, are known to cause appreciable number of unit devices useless.
The incomplete wetting between electrolyte and the wafer is a surface phenomenon and can be results of many process parameters that affect the surface properties of the electrolyte and the wafer. To name a few, the compatibility between the wafer surface and the electrolyte that contain various types of the organic additives, the environment the wafer is exposed to prior to electroplating or electroless plating, the age of the plating electrolyte, the queue time between the current deposition step and the previous step, which is either implementing a thin metal layer or polishing to expose both metal and dielectric partitions, in the process flow. The queue time is especially important because oxidation of wafer surface during this time significantly changes the surface properties of the wafer.
One method to improve wetting for electroplating of copper is to implement a pre-wetting station on an electrochemical deposition equipment, as disclosed by US 2004/0069644, and US 2007/7223323. According to this method, a wafer is first wetted in a spin-rinse-dry (SRD) module with de-ionized wafer (DIW). A combination of spin RPM and DIW flow rates is required to improve water coverage on the wafer surface. After pre-wetting, the wafer surface is presumably to carry a thin layer of wafer when it immerses into a plating electrolyte solution. Adding pre-wetting stations to an integrated processing tool increases the overall footprint and requires lager clean room space for such an equipment to be installed.
Another method to create such a pre-wetting layer is to use an in-situ DIW rinse nozzle within the plating process module. According to this method, DIW is injected onto wafer surface while it is held on a spinning wafer chucking device facing to the plating electrolyte solution. Centrifugal force keeps DIW on the spinning wafer surface from falling into the electrolyte solution. US 2006/7146994 has disclosed such a method. This method does not require additional space, thereby the overall tool footprint unchanged; however, it is less robust as a small misalignment of DIW nozzle can lead to DIW being injected into electrolyte solution.
Both methods have three inherent problems: (1) the layer of water that each wafer carries into the electrolyte solution causes global dilution of plating bath in a high volume factory, thereby altering the pre-set process conditions, (2) the layer of water that a wafer carries into the electrolyte solution causes local dilution near the wafer surface, especially in DIW-occupied via and trench features that are to be metallized, and results voids and miss-filled features after plating, and (3) longer overall process time. It is well understood that a typical SRD process leaves a layer of water of micrometers thickness on wafer surface, as the viscous resistance to wafer flow in a thin layer is too high (inversely proportional to the cube of its thickness) for it to be spun off. The thickness of the wafer layer can be reduced by allowing evaporation; however, more process time must be added and the overall equipment throughput surfers further. Consistent evaporation process for every wafer is difficult to achieve as robot movement and pick-up/drop-off priority in an integrated equipment impacts both evaporation conditions and time.
Yet another method is to modify the plating electrolyte by adding wetting enhancement agents, typically a surfactant or a group of surfactants to lower the surface tension of the electrolyte, as disclosed in US 2004/6776893. An alternation is to pre-treat wafer surface with liquid containing these wetting agents in a separate module prior to exposing the wafer to the plating electrolyte, as disclosed in U.S. Pat. No. 6,875,260. This type of methods increase the level of processing complexity and significantly adds costs for process characterization and monitoring. As the feature size continues to shrink, pre-wetting various wafer surfaces by displacing gas in nanometer scale via trench features with a viscous liquid, as in all three methods mentioned above, is ever challenging.
U.S. Pat. No. 6,544,585 discloses a method and apparatus for producing a metal deposit inside micro-cavities formed on a surface of a substrate. The method utilizes cooling the substrate to a temperature lower than a dew point of a condensable gas to form droplets thereof within the micro-cavities. When the substrate is immersed in the plating solution, the plating solution replaces the droplets and infiltrates the micro-cavities by affiliating with the droplets of the condensable gas. However, cooling the substrate to below the dew point of the condensable gas does not always produce droplets because the condensable gas may be in a supercooled state and remain in the gaseous state. For solving this problem, the method includes a step of vibrating the substrate, which complicates the method.